Electronic-structure modulation transistor

ABSTRACT

An electronic structure modulation transistor having two gates separated from a channel by corresponding dielectric layers, wherein the channel is formed of a material having an electronic structure that is modified by an electric field across the channel.

RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 61/154,678 (entitled ELECTRONIC-STRUCTURE MODULATION TRANSISTOR,filed Feb. 23, 2009) which is incorporated herein by reference.

GOVERNMENT FUNDING

This invention was made with Government support under Grant NumberEEC-0646547 awarded by NSF. The United States Government has certainrights in the invention.

BACKGROUND

Common field effect transistors (FETs) utilize an insulated gate thatcreates a channel between a source and a drain. FETs rely on a band edgeshift using an applied gate voltage to create the channel. Other typesof transistors have also been explored. For example, velocity/mobilitymodulation transistors rely on the real-space transfer of carriersbetween two adjacent materials with different mobilities. Similarly,quantum modulation transistors (QMT) are based on the constructive anddestructive interference of the wavefunctions in the channel byelectrically changing the T-shaped box dimensions. Furthermore, quantumeffects in various planar heterostructures based on the modulation-dopedfield-effect transistor (MODFET) principle have been explored, where thefield-effect is used to perturb the barriers for carriers flowingbetween the source and the drain electrodes. The localization of thestate near band edges due to disorder in the Anderson localization isalso a relevant concept, which leads to a mobility edge, but this effectis limited by the thermal limit.

SUMMARY

An electronic structure modulation transistor having two gates separatedfrom a channel by corresponding dielectric layers, wherein the channelis formed of a material having an electronic structure that is modifiedby an electric field across the channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block cross section diagram of an electronic-structuremodulation transistor according to an example embodiment.

FIGS. 2A and 2B are block cross section diagrams of electronic-structuremodulation transistors according to example embodiments, showinggraphene nanostructures and organic molecules as the channel.

FIG. 2C shows graphs that illustrate the effect of an electric field onthe bandwidth (BW) of a localized midgap or near-midgap state of achannel according to an example embodiment.

FIG. 3 is a representation of different bandwidths illustrating ON andOFF states of an electronic-structure modulation transistor according toan example embodiment.

FIG. 4 illustrates enhancement and depletion mode symbols for anelectronic-structure modulation transistor according to an exampleembodiment in enhancement mode and depletion mode operations.

FIG. 5 is a block cross section diagram of a portion of anelectronic-structure modulation transistor according to an exampleembodiment for a graphene nanoribbon with armchair edges along withgraphs illustrating energy distribution at different electric fieldstrengths.

FIG. 6 is a block cross section diagram of a portion of anelectronic-structure modulation transistor according to an exampleembodiment for a graphene nanoribbon with zigzag edges.

FIG. 7 is a graph illustrating atom number versus the absolute value ofwavefunction ψ for the channel of an electronic-structure modulationtransistor according to an example embodiment.

FIG. 8 illustrates the bandwidth of the channel in meV response to gatevoltage differential in mV to create an electric-field inside thegraphene. nanoribbon according to an example embodiment.

FIG. 9 shows the transfer characteristics (drain current versus gatevoltage) of the transistor showing high ON/OFF ratio under small supplyvoltage requirements for various drain voltages.

FIG. 10 shows the output characteristics (drain current versus drainvoltage) of the transistor showing negative differential resistancefeature under small supply voltage.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration specific embodiments which may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that structural, logical andelectrical changes may be made without departing from the scope of thepresent invention. The following description of example embodiments is,therefore, not to be taken in a limited sense, and the scope of thepresent invention is defined by the appended claims.

Electronic-structure modulation transistors (EMT) are described thatutilize one, two or three dimensional channels. In contrast to theconventional field-effect transistor (FET) that rely on the band edgeshift using an external gate voltage, in electronic-structure modulationtransistors, the gate voltage modifies the band dispersion and hencemodulates the bandwidth. In some embodiments, electronic-structuremodulation transistors operate with a few kT of supply voltage withmodulation of the bandwidth of a state accomplished by using an externalgate voltage.

Such electronic-structure modulation of bandwidth may not only beachieved by changing the coupling between lattice sites, but also bytaking advantage of the k-dependent and gate voltage dependentreal-space localization of a state. In one example, an N=9 zigzaggraphene nanoribbon is used to provide a channel exhibitingelectronic-structure modulation of its bandwidth. In another example, anarmchair graphene nanoribbon is used to provide a channel exhibitingelectronic-structure modulation of its bandwidth.

FIG. 1 is a block cross section diagram of an exampleelectronic-structure modulation transistor 100. Electronic-structuremodulation transistor 100 includes a source 110 and drain 115 separatedby a channel 120. Two gates 125, 130 are positioned either side of thechannel 120 and are separated from the channel by respective dielectriclayers 135, 140. The gates 125, 130 may be referred to as top and bottomgates, left and right gates, or side gates depending on the orientationof the electronic-structure modulation transistor 100. The gates 125,130 operate similar to capacitor plates, forming an electric fieldacross channel 120 when a difference in voltage is applied to the gates.In various embodiments, the gates may be respectively coupled todifferent voltages, one or both of which are varied to modulate theelectric field across the channel 120. One of the gates may be coupledto a ground.

In one embodiment, the gates are formed of metal, or highly dopedsemiconductor material such as silicon doped beyond degeneracy toprovide high conductivity. If metal is used, it may be selected to becompatible with the material used for the dielectric layers 135, 140,which may be SiO₂, or other insulating material. Typical metals includechromium, gold, titanium and platinum. In one embodiment, the gates,dielectric layers and channel are formed in a vertical stack supportedby a semiconductor substrate with insulating dielectric.

Various channel materials, such as molecule and graphene nanostructuresmay be used that exhibit an electronic structure change responsive toelectric fields across them as shown in FIGS. 2A and 2B. Channel 120 beformed of one-dimensional, two-dimensional, and three-dimensionalnanomaterials in various embodiments. In one embodiment, styrenemolecules, such as styrene chains may be utilized as shown at channel222 (see FIG. 2B). In further embodiment, graphene nanostructures, suchas nanoribbons and disordered graphene may be used as shown at channel220 (see FIG. 2A). Still further materials, such as multi-molecularwires, nanowires of various materials such as silicon, nanoribbons ofvarious materials, organic molecules and C60 are among some of thematerials which exhibit electronic structure modulation in an electricfield. In one embodiment, the channel 120 is formed of nanowires of Sior other semiconducting materials where a near-midgap state can bemodulated. As indicated in FIG. 1, various channel structures, such as aone, two, and three dimensional channel may be formed of suitablematerials. In further embodiments, electronic-structure modulationtransistors may be formed horizontally on a supporting substrate withinsulating dielectric, with gates and dielectrics laterally positionedon either side of a channel.

FIG. 2C includes graphs that illustrate the effect of an electric field250 on bandwidth 260 of a localized midgap or near-midgap state of thechannel, whereas the bandwidths of the source and drain contacts do notchange. As the electric field increases, the bandwidth also increases,directly affecting the conductivity of the channel as shown in FIG. 3.The representations in FIG. 3 illustrate that small bandwidth (BW) willlead to small conduction between source and drain, which accounts for anOFF state. Large bandwidth (BW) results in higher conduction betweensource and drain, which results in ON state of the transistor. Tunnelthrough higher lying bands is small.

In operation, a first voltage is applied across two gates having achannel disposed between the two gates to modify an electronic structureof the channel. A second voltage is applied across the two gates tomodify the electronic structure of the channel differently than themodification of the electronic structure caused by the first voltage.The electronic-structure modulation transistor 100 may be operated inboth enhancement mode and depletion mode as indicated by correspondingsymbols 470, 480 in FIG. 4 by changing the voltages applied across thegates.

FIG. 5 is a block cross section diagram of a portion of anelectronic-structure modulation transistor 500. For simplicity, thesource and drain are not illustrated, but would be present on eitherside of the channel in various embodiments. Electronic-structuremodulation transistor includes a top gate 510, top dielectric 515,channel 520, bottom dielectric 525 and a grounded gate 530. The channelin one embodiment is formed of a 1 nm wide armchair graphene nanoribbon550 with disordered edges. The bandwidth of the midgap state is quitesmall without an electric-field. By applying an electric-field of 1V/nm,bandwidth can be increased to about 0.3 eV as illustrated in graphsbelow the transistor.

FIG. 6 is a block cross section diagram of a portion of anelectronic-structure modulation transistor 600. For simplicity, thesource and drain are not illustrated, but would be present on eitherside of the channel in various embodiments. Electronic-structuremodulation transistor 600 includes a top gate 610, dielectric 615,channel 620, bottom dielectric 625 and a grounded gate 630. Channel 620in one embodiment is formed of a N=9 zigzag graphene nanoribbon.

FIG. 7 is a graph illustrating atom number versus the absolute value ofwavefunction ψ for the channel 620.

FIG. 8 illustrates the bandwidth of the channel in meV response to gatevoltage differential in mV to create an electric-field inside thegraphene.

FIG. 9 shows the transfer characteristics (drain current versus gatevoltage) for various drain voltages. Very high ON/OFF current ratios areobserved with small gate voltage change—thus satisfying the small supplyvoltage goal.

FIG. 10 shows the output characteristics (drain current versus drainvoltage) for various gate voltages. A negative differential resistancefeature is observed. Thus the output resistance depends on the load linein a way that the output resistance can be very high if operated aroundthe inflection point. Such high output resistance of theelectronic-structure modulation transistor can lead to gain, which is animportant requirement for use of transistors in logic operations.

In various embodiments, electronic modulation transistors may exhibitgain, low power consumption such as a V_(DD) of a few kT, and scalingbeyond 10 nm. In various embodiments, electronic modulation transistorsmay exhibit gain, low power consumption such as a V_(DD) or a few kT,scaling beyond 10 nm and pico-second operation due to all-electronicoperation. Pico-second operation facilitates nanosecond operation at acircuit level consistent with GHz processor speed.

Graphene Nanoribbon Transistor Fabrication:

The fabrication of zigzag and armchair graphene nanoribbon transistorsin side-gated geometry having imperfect edges with either metalliccontacts or graphene contacts is now described. Wafer-scale graphenefilms may be grown using chemical vapor deposition with methane gas onnickel substrate at 1000° C. and subsequent transfer on a siliconsubstrate with 300 nm thermal oxide for contrast imaging to opticallydetermine the number of layers. In a further method, graphene grown onSiC substrate may be transferred and used. Spatially-resolved Ramancharacterization may be further performed. A raster scan of 1 μm spatialresolution may be used to ensure determination of the number of layerson micron scale of lateral resolution. Although a single layer isutilized in one embodiment, multilayer graphene nanoribbons may alsoshow similar behavior due to multiple weakly coupled conductionchannels.

After material characterization, electron beam lithography may beperformed using a bi-layer PMMA (Polymethylmethacrylate (Acrylic))process to achieve 20 nm features with about 20 nm stitching accuracy.Over-etching (plasma) may be used to obtain few nm wide nanoribbons. Asecond step of electron beam lithography may be used to define featuresin side gates of about 30 nm separation (so that gate dielectricthickness is about 15 nm) with the nanoribbon of few-nm width in themiddle and source drain contacts (either graphene or metallic contacts)separated by few tens of nm to 1 μm length scales for characterizationof channel length.

Contact pads that are 1 μm sized may be fabricated usingphotolithography and matched with features on nm scale using alignmentmarks. Gate dielectric may be deposited using atomic layer deposition.HfO₂ or other material may be used for high-K gate stack formation,which will help achieve an effective-oxide-thickness of about 2-3 nm.Finally, an etch through the oxide may be performed for making contactwith source, drain and gate electrodes.

Molecular Transistor Fabrication:

The fabrication of transistors having molecular wires of styrene orother organic molecules with a channel defined either by break junction(to obtain sub 10 nm feature size) or electron beam lithography (toobtain 20 nm feature size) on undoped silicon substrate is nowdescribed. Once the channel is lithographically defined, styrene orother organic molecules may be deposited by a well-established 2Dself-assembly technique through cyclo-addition on Si(100) surface, whichwill form 1D wires in parallel with a spacing of about 7.68 Å. A topgate dielectric may be deposited first by gentle physical vapordeposition of 2 nm SiO₂ and then atomic layer deposition of 10 nm HfO₂or other high-k dielectric material to have anequivalent-oxide-thickness of about 4 nm. Top gate photolithography andcontact opening is performed to have the final device structure withmolecules in a 20 nm gap. Similar process flow may be carried out forbreak-junctions, where the electron beam lithography would be replacedby an electrical break junction formation method.

Working principles as they are currently understood are now described byuse of formulas. No representation is made as to the correctness of theprinciples described, and they are presented merely to supplement theabove description of the EMT.BW=Mag(α|eV_(g) |+BW ₀); ansatz

α=Modulation factor (dimensionless)

α≈0.9 and BW₀ is zero for the zzGNRBW=|4t ₀|

t₀=±α|eV_(g)|/4+BW ₀/4

\

T_(non-equilibrium)(E) is calculated from NEGF.

$I = {\frac{2e}{h}{\int{{\mathbb{d}E} \cdot {{T(E)}\left\lbrack {{f_{1}(E)} - {f_{2}(E)}} \right\rbrack}}}}$

The Abstract is provided to comply with 37 C.F.R. §1.72(b) is submittedwith the understanding that it will not be used to interpret or limitthe scope or meaning of the claims.

The invention claimed is:
 1. An electronic structure modulation transistor having two gates separated from a channel by corresponding dielectric layers, wherein the channel includes graphene nanoribbon with imperfect edges and forms an electronic structure that has a near-midgap state, wherein a band width of the near-midgap state of the channel is modifiable by an electric field across the channel.
 2. The transistor of claim 1 wherein the band width of the near midgap state of the channel is modulated by the electric field.
 3. The transistor of claim 1 wherein the channel is formed of zigzag graphene nanoribbon.
 4. The transistor of claim 1 wherein the channel is formed of molecular nano structures.
 5. The transistor of claim 1 wherein the channel includes nanowires of Si or other semiconducting materials where a near-midgap state can be modulated.
 6. The transistor of claim 1 wherein the gates are formed of conductively doped silicon.
 7. The transistor of claim 1 wherein the gates are formed of conductive metal.
 8. The transistor of claim 1 wherein the gates, dielectric layers and channel are formed in a vertical stack supported by a semiconductor substrate with insulating dielectric.
 9. The transistor of claim 1 wherein the gates, dielectric layers and channel are formed in a horizontal arrangement supported by a semiconductor substrate with insulating dielectric.
 10. The transistor of claim 1 wherein the electronic structure of the channel is modulated by a voltage applied across the gates.
 11. The transistor of claim 1 wherein the channel is formed of armchair graphene nanoribbon.
 12. An electronic structure modulation transistor having two gates separated from a channel by corresponding dielectric layers, wherein the channel includes styrene chains or other molecular chains and forms an electronic structure that has a near-midgap state, wherein a band width of the near-midgap state of the channel is modifiable by an electric field across the channel.
 13. A transistor comprising: a pair of gates having adjacent dielectric layers; a channel disposed between the adjacent dielectric layers, wherein the channel includes graphene nanoribbon with imperfect edges and forms an electronic structure that has a near-midgap state, wherein a band width of the near-midgap state of the channel is modifiable by an electric field provided from a voltage applied across the pair of gates.
 14. The transistor of claim 13 wherein the bandwidth of the near-midgap state of the channel is modulated by the electric field.
 15. The transistor of claim 13 wherein the channel is formed of zigzag graphene nanoribbon.
 16. The transistor of claim 13 wherein the channel is formed of molecular nano structures.
 17. The transistor of claim 13 wherein the channel is formed of nanowires of Si or other semiconducting materials where a near-midgap state can be modulated.
 18. The transistor of claim 13 wherein the gates are formed of conductively doped silicon.
 19. The transistor of claim 13 wherein the channel is formed of armchair graphene nanoribbon.
 20. A transistor comprising: a pair of gates having adjacent dielectric layers; a channel disposed between the adjacent dielectric layers, wherein the channel includes styrene chains or other molecular chains and forms an electronic structure that has a near-midgap state, wherein a band width of the near-midgap state is modifiable by an electric field provided from a voltage applied across the pair of gates. 